The present invention relates to a process for forming a coating on conductive particles by grafting a polymer and/or copolymer onto these particles using a bath containing at least one monomer from which the polymer and/or copolymer is formed. The expression xe2x80x9cconductive particlexe2x80x9d applies equally well to powders, granules and short fibres as to objects of any shape and relatively small size.
The production of composites comprising a polymer matrix in which electrically conductive particles are uniformly dispersed is limited at the present time by the instability of the conductive filler/polymer interface. To alleviate this drawback, several processes involve precoating the fillers with an organic component which will ensure, at least partially, that the particles are bonded to the polymer matrix of the composite.
In fact, this approach provides only a very limited solution to the stability of the final composite since the problem of conductive particle/polymer matrix adhesion is transferred to the conductive particle/coating layer interface.
An improvement to the known processes has in particular been described in 1995 by N. Tsubokawa and S. Hayashi (J.M.S.xe2x80x94Pure Appl. Chem., 1995, A32, 525-535) which relates to the chemical grafting of polymers onto a carbon powder preoxidized by nitric acid, so as to form OH, COOH and Cxe2x95x90O functional groups on the surface. The grafting of the polymer takes place in two steps: grafting of the functional group, then chemical bonding of the polymer. The grafting of the polymer is not controllable since the density of grafted chains is limited by the surface density of the functional groups, which is random moreover. The attachment of the polymer is not really stable since, in some cases, it is the hydrolysable xe2x80x9cesterxe2x80x9d functional groups which provide the graphite/polymer bond. Finally, these authors use either monomers whose polymerization is initiated by the grafted functional group, or already formed polymer chains whose size limits the accessibility to the substrate.
One of the essential aims of the present invention is to remedy the drawbacks of the known coating processes and to present a process making it possible to form a coating on electrically conductive particles which adheres perfectly and uniformly, so that these particles thus coated or encapsulated are particularly suitable for the manufacture of composites filled with such conductive particles.
The object of the process according to the invention is therefore to cover the particles with a stable polymer layer capable of dispersing them in a stable and homogeneous manner in a conductive or non-conductive polymer matrix provided that, of course, measures are taken to ensure that this matrix and the polymer coating the particles are miscible, or even mutually reactive.
For this purpose, according to the invention, the grafting is carried out by electrochemical reduction of the said monomer in an electrolysis bath in which at least one cathode and one anode are provided and which contains an aprotic solvent, an electrolyte support and the monomer or monomers required to form, by polymerization or copolymerization, the coating, by putting the particles into suspension in the abovementioned bath and by making them move therein, so as to create intermittent contact between the said particles and the cathode, and to form, by polymerization or copolymerization, the coating on the particles by applying a potential or a current which drives the reaction system into a passivation region corresponding to the inhibition peak detected by voltammetry, the cathode remaining insensitive to the said polymerization or copolymerization during application of the abovementioned potential or current.
Further details and features of the invention will emerge from the description given below, by way of non-limiting examples, of a few particular ways of implementing the process according to the invention, with reference to the appended drawings.